1. Field of the Invention
The present invention relates to a plasma display panel (PDP) driving method and apparatus, and, in particular, a PDP driving method and apparatus that prevents subfield position variation by arranging an idle period among groups of subfields in a frame.
2. Description of the Related Art
Generally, a PDP is driven by frames that are divided into subfields. These subfields may include a sustain period having a respective weight value, and the PDP displays gray-scale data as brightness according to the combination of the weight values of each subfield. When the number of subfields included in a given frame is increased, the brightness generated by each subfield may be reduced, which allows improved control of gray-scale data display. In addition, false contours may be prevented because the brightness difference among subfields is reduced.
However, for use as a television (TV), it is preferable that a PDP provide sufficient luminescence. A typical light emitting efficiency of a PDP is of about 1-3 lm/W.
The brightness of the PDP is principally dependent on the number of sustain pulses used to provide a sustain discharge during a frame. Generally, in order to achieve sufficient brightness, 1,400 to 3,000 sustain pulses are used each frame to acquire a peak luminescence of 650-1,500 cd/m2. As a result, a sufficient number of subfields may not be used for expressing gray-scale data. A typical number of subfields used in a PDP TV is about 10 to 16 subfields.
Referring to FIG. 1, in a 6-bit gray-scale data display scheme, one TV frame is divided into 6 subfields SF1-SF6. Each subfield may be divided into an address period A1-A6 and a sustain period S1-S6. An idle period R is placed at an end of the frame, in which no discharge occurs. The idle period R is given as a residual time of the frame excluding the time for the subfields.
The subfields will be now described in reference to an image standard of the National Television System Committee (NTSC).
According to the NTSC standard, 60 frames are included in one second and, thus, 16.67 ms may be used for realizing a respective frame.
As noted above, 10-16 subfields are included in a TV frame, and each subfield may include a reset period, an address period, and a sustain period. The reset period is about 200 μs, and the address period can be determined by multiplying the number of scan lines by a scan pulse width. For example, in an standard definition PDP, in which the number of scan lines is 480 and the scan pulse width is 1.7 μs, the address period is 816 μs (480 lines×1.7 μs). Since sustain periods have different weight values, there may be a different number of sustain pulses throughout the subfields' sustain periods. One sustain pulse period usually takes about 5 μs. The reset period and the address period, however, are typically uniform throughout the subfields.
A typical maximum brightness of a PDP TV is about 1000 cd/m2. Therefore, in order to realize higher brightness levels, the efficiency of the PDP or the number of sustain pulses must be increased. However, since a significant number of the sustain pulses are already being used, their number may not be easily increased.
Recent PDP TV's mostly use 10-12 subfields per TV frame, although the number of subfields could be increased in a more efficient PDP. A variable subfield scheme that varies the number of subfields according to an average signal level (ASL) of an image is now being used to efficiently control the number of subfields according to brightness. The ASL, which is an average signal level of an image data histogram or a load ratio, can be given by the following Equation 1.
                    ASL        =                                            (                                                                    ∑                    V                                    ⁢                                      RDATA                    n                                                  +                                                      ∑                    V                                    ⁢                                      GDATA                    n                                                  +                                                      ∑                    V                                    ⁢                                      BDATA                    n                                                              )                        /            3                    ⁢          N                                    (                  Equation          ⁢                                          ⁢          1                )            
Here, V indicates one frame.
Because high power consumption is a PDP driving variable, an automatic power control (APC) scheme may also be used to control power consumption based on the ASL (or the load ratio) of a frame to be displayed. According to the APC scheme, power consumption is kept below a predetermined level by varying an APC level in accordance with a load ratio of input image data, and by varying the number of sustain pulses in accordance with the APC level.
As shown in FIG. 2, the APC is shown to include only three stages for demonstrative purposes. However, an APC may be realized with a much greater number of stages, such as 128 or 256 stages, for example.
APC stage 0 is utilized when an image of a low ASL is input from the outside, i.e., the input image is dark or requires only a small screen area to display. The number of sustain pulses provided is relatively large because little power is consumed. In contrast, APC stage 2 is utilized when the image is bright or requires a large screen display area. Accordingly, the power consumption is high and the number of sustain pulses provided is decreased to limit power consumption. Therefore, an idle period is enlarged in APC stage 2, because the time for the subfields in a frame is relatively shorter than those of APC 0 or APC 1.
False contours, however, may result due to the variances in idle periods between TV frames using different APC stages. Therefore, in order to reduce these false contours, such an APC scheme may be combined with a variable subfield scheme.
A conventional variable subfield scheme may be found in Korean Patent Publication No. 10-2000-0070527. This reference discloses that gray-scale data is displayed by selecting 11 or 12 subfields depending on the APC level, which in turn depends on an ASL or a load ratio of displayed image. When displaying a dark image of a low APC level, the gray-scale data is usually displayed with 11 subfields for maximum brightness.
When displaying a bright image of a high APC level, sustain discharges are generated in a large number of discharge cells of a PDP, which increases the power consumption of the PDP. Therefore, the number of sustain discharges needs to be limited in order to maintain the power consumption at a predetermined level. Moreover, the false contours occur more often in the case of displaying a bright image rather than a dark image. Accordingly, the gray-scale data is displayed using an increased number of subfields (i.e., 12 subfields), which reduces the number of sustain discharges in the subfield, in part, because of the increased time taken by the additional reset and address periods. According to the conventional art, when the number of subfields is changed according to the variable subfield scheme, a center of the subfield of an uppermost weight value is changed and an abnormal variation is caused in image brightness. In order to prevent such a phenomenon, the idle period is placed foremost in the frame.
The abnormal image brightness variation and flicker may be prevented to some degree by placing the idle period foremost in the frame. However, the weight value allocated to each subfield is greater in a frame having a lesser number of subfields than a frame having a greater number of subfields. Therefore, the light emitting time of the rest of the subfields is changed even if a finishing point of the subfield having a maximum weight value is the same. As a result, flicker still occurs when the number of subfields in a frame is changed.